In conventional integrated circuit technology, it is known to provide a layer of polysilicon on top of a thick field oxide or shallow trench isolation (STI) as a substrate for building electrical components such as polysilicon resistors, electrical fuses (eFuse) and thin film transistors (TFTs). Precision resistors are used extensively in analog and mixed signal circuits including current mirrors, analog-to-digital and digital-to-analog converters and switched capacitor filters. Polysilicon eFuses are cheap and efficient one-time-programmable devices available in traditional semiconductor process technologies. The aforementioned polysilicon layer is typically the same polysilicon used to form the gate electrode of the field effect transistor (FET).
However, there are drawbacks to forming electrical devices on top of the field oxide or STI using the same material layer as the FET gate. In particular, the advent of the metal gate field effect transistor has created challenges for fabrication of polysilicon resistive elements, such as a precision resistor. Various factors, such as reduced gate height, metal gates, High-K dielectrics, and additional mask requirements complicate the process. In particular, the metal gates used in advanced integrated circuit FETs are often not suitable for precision resistor or fuse applications because the resistance of a metal cannot easily be tuned to specific values. Therefore, it is desirable to have an improved structure and method for fabrication of precision resistive elements comprised of polysilicon.
Electronic fuses, also known as eFuses, are a special case of a resistive element. Analogously, polysilicon eFuse implementation with reduced gate height becomes challenging due to difficulty in maintaining optimal thicknesses of silicided and unsilicided regions. Alternate implementations of eFuse in bulk substrate have been proposed. Bulk substrate implementation has several drawbacks, such as; (a) insufficient difference in fuse resistance between programmed and unprogrammed states; and (b) heat loss through conductive substrates makes it difficult to maintain temperature gradient across the fuse during programming and thus much higher voltages are required. Thus, it is desirable to have an improved structure and method for fabrication of eFuse devices.